The present invention relates generally to lasers and more particularly to a multimode laser.
In optical fiber communication systems with bit rates up to about 200 Mb/s, graded-index fibers are preferably used. The light sources preferably employed in such systems are multimode lasers, because single-mode lasers, due to their long coherence lengths, tend to generate excess noise as a result of backscatter from the fiber. Such multimode lasers, particularly for the wavelength region around 850 nm, have been fabricated on the "V-groove principle". This principle has also been used for the wavelength region above 1000 nm. It requires a precisely controlled diffusion, which can only be achieved after costly experimentation.
Conventional oxide-stripe lasers with stripe widths ranging from 15 to 20 um cannot be used because they tend to cause changes in the near field and, thus, intensity variations in the current pulse. A way out was proposed by Dr. Renner and G. Henshall in February 1981 in the "IEEE Journal of Quantum Electronics", Vol. QE-17, pp. 199 to 202, namely to reduce the width of the stripe opened in the insulating oxide layer to a few micrometers. In this manner, stable multimode behavior is achieved, but thermal problems arise. As a result of the narrower contact area, the flow of current is concentrated. If the contact areas are less than 10 um wide, the threshold current of the laser increases with decreasing contact width. The higher threshold current and the more concentrated current flow result in more heat being produced during operation of the laser. The removal of this heat is obstructed by the narrower window in the oxide. Since the properties of semiconductor lasers are strongly temperature-dependent, the temperature rise resulting from the aforementioned disadvantages is undesirable.